A magnetic phase shifter has, in spite of its simple construction, such an advantage that it can compare a plurality of control inputs in a relation insulated from one another and can convert the result of comparison into an amplified phase-shifted output having a considerably well shaped waveform. Also, the magnetic phase shifter has such other advantages that its time constant can be suitably adjusted, and its operation is hardly affected by noise. Therefore, a chopper control system, which is simple in construction and can operate with high reliability, can be realized when a magnetic phase shifter is used for the gate control of a semiconductor chopper such as a thyristor chopper.
In an electric vehicle or the like driven by a DC motor controlled by a semiconductor chopper, an instruction signal for the current value to be supplied to the DC motor is produced according to the amount of depression of the accelerator pedal, and applied to a current control system in the vehicle which includes generally a feedback path for the negative feedback of the actual value of the motor current. More particularly, when a magnetic phase shifter is used for the gate control of the chopper, the motor current instruction signal is applied to its first control winding, and the actual motor current signal is applied to its second control winding, so that the duty factor of the chopper can be adjusted depending on the difference between the magnetomotive forces induced by these signals in the first and second control windings of the magnetic phase shifter.
In such a chopper control system, various modes of control based on the load current signal are also frequently required other than the above mode of current control. For example, when the duty factor of the chopper attains 100%, the chopper is short-circuited through a bypass contactor for preventing an excessive rise in the temperature of the thyristor or like semiconductor element in the chopper. Under such a condition, however, if the electric vehicle were subjected to a great decrease in the motor speed in order, for example, to ascend a slope way or get away from an unexpected pit, the motor current would increase greatly, resulting in damage of the motor by burning due to excess temperature rise. To avoid the above trouble, the bypass contactor should be released as soon as the motor current value increases up to a predetermined setting. For this purpose, it is required to detect the actual value of the motor current. Also, when the electric vehicle running under a light-loaded condition is to be driven at a higher speed, a control mode called field-weakening control similar to that applied to electric railway vehicles should take place upon detection of the decrease in the motor current value.
However, it is not so simple to detect actually the value of the motor current. This is because the control circuit is frequently required to be isolated or insulated from the main circuit, and elements including a DC transformer and an overcurrent relay are additionally required for the current detection purpose, resulting in an uneconomical control system. Especially, in the case of an electric vehicle or a vehicle such as a battery-driven forklift truck, the additional provision of the elements above described is economically very difficult and is also undesirable from the viewpoint of the available space.